1. Field of the Invention
The present invention relates to an audio signal processing circuit, and more particularly to an audio signal processing circuit which is composed of thin film semiconductor elements. Further, the present invention relates to a display device comprising the audio signal processing circuit.
2. Description of the Related Art
In recent years, mobile phones have been widely used with the advance of the communication technology. In the future, transmission of moving images and transmission of a larger amount of information are expected. On the other hand, by reducing weight of personal computers, those adapted for mobile communication have been produced. The information terminal called a PDA originated in electronic notebooks has also been produced in large quantities and is expected to be widely used. In addition, most of those portable information equipment are equipped with a flat panel display with the development of display devices.
Additionally, in recent years, in active matrix display devices, manufacturing of a display device for which a thin film element (typical example is a thin film transistor; hereafter referred to as a TFT) using a polycrystalline semiconductor crystallized by low temperature is used has been promoted. The low temperature mentioned above means that a crystallization temperature of 600° C. or less is lower compared to a conventional crystallization temperature of 1000° C. or more. With the TFT using the polycrystalline semiconductor crystallized by low temperature, as well as a pixel, a signal driver circuit can be integrally formed around a pixel portion. Thus, it is possible to realize a compactness and a high definition of a display device and it is expected to be more widely used in the future.
On the other hand, for the portable information equipment, other output functions, particularly an audio output function is required as well as a visual display function. By displaying an image with sound, it becomes possible to view the image effectively and enjoy the image more.
A current audio output device outputs sound by converting electric signals into sound using a cone speaker or the like. Since the cone speaker occupies large space in a portable information equipment, reduction in size and weight of the portable information equipment has been prevented.
FIG. 2A is a top plan view of a periphery of a display device in a portable information equipment 201 with a conventional audio output function. FIG. 2B is a cross-sectional view. The display device has a substrate 209 on which a pixel portion 204, a source signal driver circuit 202, a gate signal driver circuit 203 are integrally formed. Attached to the display device are a cone speaker 207, an FPC 205, a counter substrate 208, and a printed circuit board 206 on which an audio signal processing circuit 210 and a coupling capacitor 211 are mounted.
The cone speaker 207 is not suitable for reduction in size and weight of portable information equipment due to its large size. For the above-mentioned reason, a flat speaker as shown in FIGS. 3A and 3B is being developed. FIG. 3A is a top plan view of the periphery of a display device in a portable information equipment 301 with a flat speaker. FIG. 3B is a cross-sectional view. The display device has a substrate 309 on which a pixel portion 304, a source signal driver circuit 302, a gate signal driver circuit 303 are integrally formed. Attached to the display device are a flat speaker 306, FPCs 305 and 308, a counter substrate 310, and a printed circuit board 311 on which an audio signal processing circuit 307 and a coupling capacitor 312 are mounted.
The flat speaker is similar to conventional cone speakers in that electric signals are converted into vibrations to output sound, but is different in that a glass substrate, a plastic substrate, a touch panel, or the like in the display device and the like is vibrated instead of a cone speaker. With such a flat speaker, the portable information equipment with a smaller size and lighter weight than the one using a conventional cone speaker can be realized.
In addition, using a multilayer ceramic chip capacitor as a capacitor, the case of mounting that chip capacitor on an FPC (flexible printed circuit) (e.g. Patent Document 1: Japanese Laid-Open Patent Application No. Hei 11-326937) and the case of mounting that chip capacitor on a substrate of a display device (e.g. Patent Document 2: Japanese Laid-Open Patent Application. No. Hei 7-261191) are reported. This kind of chip capacitors with a long side of approximately 2 to 3 mm has a great advantage in reducing the volume.
Although the flat speaker is a very effective mean for reduction in size and weight of portable information equipment as described above, there are problems to be solved. For the audio signal processing circuit 307 which drives the flat speaker 306, as shown in FIGS. 3A and 3B, the printed circuit board 311 is set to the external of the display device with the audio signal processing circuit 307 made of a LSI mounted thereon as a conventional audio signal processing circuit. Therefore, this is still insufficient for reduction in size and weight of a portable information equipment. In addition, since the audio signal frequency is low, the coupling capacitor 312 having a large capacitance value (for example, 10 to 100 iF) is required for coupling an audio signal with a capacitor. For this kind of capacitors, there are no other choices than an electrostatic capacitor, which has a large volume (typically a cylinder form with a diameter of approximately 5 to 10 mm and a height of 7 to 10 mm). Therefore, the volume of a circuit has been increased.
Although a multilayer ceramic chip capacitor as the one mentioned in Patent Document 1 and Patent Document 2 has smaller volume than an electrostatic capacitor, it has a problem in that the maximum capacitance value is approximately 0.1 iF at most. Thus, it is necessary to increase the resistance value of a resistor in order to constitute a coupling of an audio signal processing circuit 307 by a chip capacitor. When the lowest frequency of the audio signal is 20 Hz and the capacity is 0.1 iF, a resistance value of approximately 80 kΩ or more is required.
In the case where the resistor is used as a chip component like other components, it causes the deterioration in a frequency characteristic of the audio signal processing circuit 307. The reason is that when drawing the circuit to a printed circuit board 311 or an FPC 308, parasitic capacitances arise at terminal portions and then low pass filters are formed between the parasitic capacitances and the chip resistor, and therefore to reduce a frequency characteristic. For example, considered hereinafter will be the case of an amplifier circuit for 10 times amplification with a circuit structure as shown in FIG. 4. An audio signal processing circuit 401 includes an operational amplifier 402. An amplifier circuit is composed of the operational amplifier 402, external resistors 403, 404, and 405, external capacitors 406 and 407, a signal source 408, and a bias supply 414.
When the resistance values of the external resistors 404 and 405 are referred to as R404 and R405 respectively, a gain (1+ (R405/R404)) of the amplifier circuit is obtained in the circuit as shown in FIG. 4. On the other hand, in order to obtain a low frequency of at least 20 Hz with a chip capacitor having a capacity of 0.1 iF, it is necessary to set the resistance value of the external resistors 403 and 404 at 80 kΩ or more, as described above. Assuming here that each of the external resistors 403 and 404 is respectively set to have a resistance value of 100 kΩ with some margin, the external resistor 405 of 900 kΩ is required for obtaining a gain of 10 times as large in the amplifier circuit.
However, parasitic capacitors as the ones denoted by reference numerals 409 to 413 arisen by setting input and output terminals of the operational amplifier 402 at an external portion of the audio signal processing circuit as shown in FIG. 4. In particular, since the parasitic capacitor 410 arises in parallel with the external resistor 405 of 900 kΩ, a low pass filter is formed therebetween. When the value of the parasitic capacitor 410 reaches 10 pF, a low pass filter of 17 kHz is formed. Because of the low path filter, the high frequency side of an amplifier is limited and a problem of reducing the frequency characteristic occurs. Comparing a case 601 of having a parasitic low pass filter with a case 602 of not having a parasitic low pass filter as shown in FIG. 6, there is a remarkable difference in frequency characteristics of them.
In an audio signal processing circuit composed of an external resistor incorporated in a conventional IC using single crystal silicon, the same problem occurs. An example will be explained with reference to FIG. 5. In an IC using single crystal silicon, MOS transistors and a resistor are formed on a silicon substrate 501. In FIG. 5, 502 and 503 denote a source region and a drain region of a MOS transistor respectively, 504 denotes a channel region, 505 denotes a LOCOS (LOCal Oxidation of Silicon) film, 506 denotes a gate insulating film, 507 denotes a gate electrode, 508 denotes an interlayer film, 509 denotes a source electrode, and 510 denotes a drain electrode. As for a resistor 511 formed on the LOCOS film 505, one terminal is connected to the drain electrode 510 and the other terminal is connected to the electrode 512. Because the LOCOS film 505 is approximately 200 to 5000 nm in thickness, there is a parasitic capacitor 513 between the silicon substrate 501 and the resistor 511. The resistor of approximately 1 MΩ causes a parasitic capacitance of several pF. A silicon substrate is a conductor, and is typically connected to a GND and the like. Therefore, the same problem as the above occurs and the frequency characteristic has been deteriorated.